Bug 819443 - Make sure we don't finalize on background thread (r=luke)

js/src/jscntxt.h

@@ -482,7 +482,11 @@ struct JSRuntime : js::RuntimeFriendFields
     void clearOwnerThread();
     void setOwnerThread();
     JS_FRIEND_API(void) abortIfWrongThread() const;
+#ifdef DEBUG
     JS_FRIEND_API(void) assertValidThread() const;
+#else
+    void assertValidThread() const {}
+#endif
   private:
     void                *ownerThread_;
   public:

js/src/jsobjinlines.h

@@ -225,6 +225,9 @@ JSObject::finalize(js::FreeOp *fop)
     js::Probes::finalizeObject(this);
 
     if (!IsBackgroundFinalized(getAllocKind())) {
+        /* Assert we're on the main thread. */
+        fop->runtime()->assertValidThread();
+
         /*
          * Finalize obj first, in case it needs map and slots. Objects with
          * finalize hooks are not finalized in the background, as the class is